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Let K be a two-dimensional global field of characteristic ≠ 2 and let V be a divisorial set of places of K. We show that for a given n ≥ 5, the set of K-isomorphism classes of spinor groups G = Spinn(q) of nondegenerate n-dimensional quadratic forms over K that have good reduction at all v ∈ V is finite. This result yields some other finiteness properties, such as the finiteness of the genus genK(G) and the properness of the global-to-local map in Galois cohomology. The proof relies on the finiteness of the unramified cohomology groups Hi(K,μ2)V for i ≥ 1 established in the paper. The results for spinor groups are then extended to some unitary groups and to groups of type G2. [ABSTRACT FROM AUTHOR]

Let K be the field of fractions of a local Henselian discrete valuation ring OK of characteristic zero with perfect residue field k. Assuming potential semi-stable reduction, we show that an unramified Galois action on the second l-adic cohomology group of a K3 surface over K implies that the surface has good reduction after a finite and unramified extension. We give examples where this unramified extension is really needed. Moreover, we give applications to good reduction after tame extensions and Kuga–Satake Abelian varieties. On our way, we settle existence and termination of certain flops in mixed characteristic, and study group actions and their quotients on models of varieties. [ABSTRACT FROM AUTHOR]